There is a widespread demand for high purity xylenes, and therefore a need for effective, practical and cost effective methods of manufacturing the same. High purity mixed xylenes are used as a solvents in chemical manufacturing, agricultural sprays, adhesives, paints, and coatings. Xylene is also an ingredient in aviation fuel and gasoline, and is used as a feedstock material in the chemical, plastic, and synthetic fibre industries. Isomers of xylene are used in manufacturing various polymers. As feedstocks, o-xylene is used in making phthalic anhydride (PA); m-xylene for isophthalic acid; and p-xylene is used exclusively for making dimethyl terephthalate and terephthalic acid (DMT/TPA) which are raw materials used in the manufacture of polyethylene terephthalate (PET) used in polyester fibers, molded plastics, films, and blown beverage bottles4.
The separation of high purity p-xylene from a mixture of mixed xylenes—m-xylene, o-xylene, p-xylene and ethylbenzene—is currently performed by one of the following methods: (1) crystallisation, (2) adsorption, or (3) a hybrid crystallisation/adsorption process5. Distillation is not used due to the boiling point difference of only 2° C. between p-xylene and ethylbenzene resulting in columns with high reflux ratios and a large number of trays.
Crystallisation-based processes exploit the large freezing point difference between p-xylene and the remaining components in the mixture. The recovery value of p-xylene is limited to the eutectic point, i.e. the temperature at which a second component starts to crystallize. Typical values for recovery are between 60-65 wt % for feed streams with about 20 wt % of p-xylene6. This limitation is one of the main drawbacks of crystallisation when processing feeds with a low concentration of p-xylene7. Furthermore, low temperature crystallisation methods have other serious drawbacks, including the large amount of energy required for cooling, and the heat-transfer problems that arise as solid p-xylene coats the inner walls of a cooled crystallisation vessel.
The more recent trend has been to design hybrid processes involving a first stage based on adsorption and a second stage based on crystallisation. Often, zeolites are used as adsorbents in the separation of xylene isomers, but zeolites have their own drawbacks, since they require very specific operating conditions, such as optimal hydration levels, in order to ensure peak performance.
More recently, alternative adsorbents have been developed1′ 2′ 3′ 7′9′10. For instance, UOP LLC have developed metal-organic frameworks (MOFs) as adsorbents for xylene. US 20110420779 describes the use of the MOFs Cr-MIL-101 and Al-MIL-53 as adsorbents for adsorbing para-xylene, and Zn-MOF-5 for ortho-xylene, in their widely used simulated moving bed technology. However, the adsorption selectivity of MOFs of the prior art, particularly when used to purify xylene mixtures, is sometimes inadequate or inconsistent.
An object of the present invention is to provide alternative compounds to serve as MOFs.
A further object of the present invention is to provide alternative MOFs for purifying xylene mixtures.
A further object of the present invention is to provide MOFs with improved sorption selectivity, especially with respect to xylene mixtures.
A further object of the present invention is to provide an improved method of purifying mixtures, such as xylene mixtures.